[Note: Literature references on the following background information and on conventional test methods and laboratory procedures well-known to the ordinary person skilled in the art, and other such state-of-the-art techniques as used herein, are indicated by numbers in parentheses and appended at the end of the specification.]
Vitamin K is required for the post-translational formation of gamma carboxyglutamic acid (Gla), which is present in a number of plasma proteins that are involved in coagulation: Prothrombin, factors VII, IX, X, protein C and protein S (1,2). Gla-mediated calcium ion binding in these proteins is necessary for their association with phospholipid surfaces and is critical for their hemostatic function (3). In 1977, Prowse and Esnouf identified an additional vitamin K-dependent protein circulating in bovine plasma and named it protein Z (PZ) (4). Initially thought to represent a single chain form of bovine factor X, bovine PZ was later identified as a discrete Gla-containing protein (5,6). The human counterpart of bovine PZ was isolated in 1984 (7).
Human PZ is a 62,000 molecular weight glycoprotein that has a plasma half-life of .about.2.5 days (8). Plasma PZ levels in blood donors span a broad range with a mean concentration of 2.9.+-.1.0 .mu.g/mL in EDTA anticoagulated samples (corresponding to .about.2.6 .mu.g/mL in citrate plasma) (8). The amino-terminal half of PZ is very homologous (40-50%) to those of factors VII, IX and X, and contains a Gla-domain, two EGF-like domains, and a region which connects to a homologue of the catalytic domains present in the serine protease zymogens. In the carboxy-terminal domain of PZ, however, the region around the typical "activation" site is absent and the His and Ser residues of the catalytic triad are lacking (the Asp residue is conserved) (9,10).
McDonald et al (11) have recently reported that the kinetics of the binding of human and bovine PZ to phosphatidylcholine/phosphatidylserine (PC/PS=75%/25%) vesicles is different from that of the other vitamin K-dependent coagulation factors. The k.sub.assn (10.sup.-5 s.sup.-1 M.sup.-1) and k.sub.dssn (s.sup.-1) rate constants are 1.95 and 0.0063 for bovine PZ and 3.36 and 0.057 for human PZ. In comparison the values of these constants for bovine prothrombin are 176.0 and 1.9, respectively. Thus, the association and dissociation rate constants for bovine and human PZ are dramatically slower than those of prothrombin and the dissociation of bovine PZ from phospholipids is significantly slower than that of human PZ.